Cooling System for Superconducting Power Apparatus

ABSTRACT

A cooling system for a superconducting power apparatus including a reservoir tank for reserving a liquefied gas, a circulating pump, a heat exchanger for cooling the liquefied gas, and a circulation loop through which the liquefied gas is circulated, the superconducting power apparatus being cooled by circulating the liquefied gas in a subcooled state using the circulating pump. The cooling system further includes a pressurizing mechanism for pressurizing the liquefied gas in the reservoir tank with the same type of gas as the liquefied gas, wherein a liquid level in the reservoir tank for reserving the liquefied gas in a pressurized state is located above an outlet of a return piping of a circulating liquefied gas at least by a dissolving depth of the pressurizing gas+(plus) a liquid level movement correction amount.

FIELD OF THE INVENTION

The present invention relates to a cooling system for cooling asuperconducting cable, a superconducting bus line, SMES, asuperconducting transducer, etc. which can be used in an industry in asuperconducting state by cooling with a liquefied gas such as a liquidnitrogen or the like and particularly to a cooling system for thecooling a superconducting power apparatus driven in a high voltagestate.

BACKGROUND OF THE INVENTION

As one of the superconducting power apparatuses, a prior art will bedescribed referring to FIG. 6 using a superconducting cable as anexample, which uses a liquefied gas such as a liquid nitrogen forcooling. A cooling system for a superconducting cable described inJapanese patent Laid-Open No. 08-148044 is known.

As shown in FIG. 6, in a conventional cooling system, a circulationcycle is repeated that a liquefied gas in a subcooled state (a statewhere the liquefied gas is cooled at a temperature lower than asaturation temperature of the liquefied gas) from a reservoir tank 101is pressurized by a pump 105 and supplied to a cable 111 after beingcooled by a heat exchanger 107 of a refrigerating machine 108 andreturned to the reservoir tank 101 again.

In cooling of the superconducting cable, when the circulated liquefiedgas is brought into a gas-liquid mixed state, a pressure loss isincreased and a required amount of the liquefied gas can not be stablycirculated, which requires a large-sized circulating pump with a largecapacity.

Moreover, since a superconducting cable employs an extremelylow-temperature electric insulating method for maintaining a highelectric insulating performance by impregnating the liquefied gas withan insulator, if a gas or an air bubble is mixed in a liquefied gas,there is a problem that an electric insulating performance is remarkablylowered.

Therefore, in order to maintain the liquefied gas in the subcooled stateall the time for circulation in a non-evaporated state in theconventional cooling system, the liquefied gas is kept not boilingduring the circulation (that is, the liquefied gas is not brought into agas-liquid mixed state).

That is, when a liquid nitrogen is used as the liquefied gas, forexample, an inside of the reservoir tank 101 is brought into apressurized state by supplying a gas such as hydrogen (H₂) or helium(He) with sufficiently lower triple point than the liquefied gas from acylinder 123 so as to raise the boiling point of the liquefied gas.

-   [Patent Document 1] Japanese Patent Laid-Open No. 08-148044

SUMMARY OF THE INVENTION

When a gas with a sufficiently low triple point than the liquefied gas,for example a liquid nitrogen as the liquefied gas, is pressurized byhelium (He) gas in a reservoir tank as in the prior art, such aphenomenon was found to occur that a slight amount of He gas isdissolved in the liquid nitrogen.

That is, helium (He) is widely known as an inactive gas and wasrecognized as non-soluble in a liquid nitrogen, but actually, it isfound out that a slight amount of He gas is dissolved in the liquidnitrogen.

The amount dissolved into the liquid nitrogen is extremely slight but ifthe liquefied gas in which He gas is dissolved is circulated, the statewhere He gas is dissolved in the liquefied gas can't be maintained andit generates air bubbles.

And the air bubbles are generated, for example, at a portion where aflow velocity of the liquefied gas is relatively lowered in a widenedpiping or at a portion where a pressure of the liquefied gas is rapidlylowered rather than that in the reservoir tank after being throttled bya valve or the like. The air bubbles are mixed in the liquid nitrogenwhich bring about a gas-liquid mixed state.

Also, if there is a portion where a superconducting cable or asuperconducting power apparatus is located higher than a cooling systemdue to its installation layout, it is found out that the generated airbubbles are collected and remain at an upper part of the equipment atsuch a portion and filled in a cooling piping of the liquid nitrogen inthe end, which prevents circulation of the liquid nitrogen.

It was made clear by inventors' experiment that the above-mentionedphenomenon occurs over an extremely long time of several months.

If a He gas is contained in the liquid nitrogen and filled in agas-liquid mixed state in a piping or filled in a gas phase in a coolingpiping, circulation of the liquid nitrogen can not be conductedsmoothly.

Moreover, since a He gas has an extremely small voltage resistantcharacteristic as compared with other liquefied gases, though a liquidnitrogen originally has a high insulating characteristic, the insulatingcharacteristic is lowered by the contained He gas, and it might cause adefective insulation or an insulation breakdown of a superconductingpower apparatus.

Taking measures against it, a pressurization of a liquefied gas in areservoir tank with the same type of gas as a liquefied gas was come upwith.

But since the liquid nitrogen reserved in the reservoir tank is with atemperature below a boiling point, when a nitrogen gas used for thepressurization contacts the liquid nitrogen below the boiling point inthe reservoir tank, the nitrogen gas is cooled and liquefied.

Therefore, the pressurized pressure is lowered, and it has a problemthat the pressure can not be kept constant unless the nitrogen gas iscontinuously supplied from a gas cylinder all the time, and as a result,a large quantity of a nitrogen gas is consumed and a large capacity ofliquefaction heat is brought into the cooling system, which increases aheat load.

Therefore, an object of the present invention is to provide a coolingsystem for a superconducting power apparatus which can circulate aliquefied gas smoothly for a long time in a subcooled state.

And in the present invention, the liquefied gas is circulated withoutdissolution of a gas, which is used for pressurization with a boilingpoint lower than that of the liquefied gas, into the liquefied gas, soas to cause an unstable factor for circulation of the liquefied gas ortroubles relating to insulation of electric apparatuses.

The inventors have conducted committed researches in order to solve theproblems of the above-mentioned prior art.

As a result, it was found out that by a pressurization with the sametype of gas as a liquefied gas in a reservoir tank, not a helium (He)gas which has been used as a pressurizing gas, a dissolution of a slightamount of He gas in the liquid nitrogen can be prevented.

By the pressurization with the same type of gas as the liquefied gas inthe reservoir tank, following problems can be solved.

That is, the He gas becomes air bubbles at a portion where a pressure ofthe liquefied gas is rapidly lowered, and a mixing thereof into a liquidnitrogen brings about a gas-liquid mixed state, which causes problems ofinability of a smooth circulation of the liquid nitrogen and adeterioration of an insulating characteristic.

Similarly, it was also found out that such a problem is solved thatgenerated air bubbles are reserved at an upper part of the apparatus,and moreover, filled in a cooling loop to prevent a circulation of theliquid nitrogen, if a difference in height of a superconducting powerapparatus due to arrangement exceeds a predetermined value.

Moreover, it was found out that following problem is solved. That is, aliquid level in a reservoir tank for reserving a liquefied gas in apressurized state is located above an outlet of a return piping of acirculating liquefied nitrogen gas, at least by a dissolving depth of apressurizing gas+(plus) a liquid-level movement correction amount.

Then the problem is solved, of which a nitrogen gas used for thepressurization is liquefied and a pressurized pressure is decreased andthe pressure can't be kept constant unless the nitrogen gas iscontinuously supplied from a cylinder all the time.

Therefore, such a problem is solved that a large quantity of thenitrogen gas is consumed and a large capacity of liquefaction heat isbrought into a cooling system at that time, which increases a heat load.

The present invention was made based on the above research results.

And a first aspect of a cooling system for a superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus comprising a reservoir tank forreserving a liquefied gas, a circulating pump, a heat exchanger forcooling the liquefied gas, a circulation loop through which theliquefied gas is circulated, in which the superconducting powerapparatus is cooled by circulating the liquefied gas in a subcooledstate using the circulating pump, and further comprising pressurizingmeans for pressurizing the liquefied gas in the reservoir tank with thesame type of gas as the liquefied gas, wherein a liquid level in thereservoir tank for reserving the liquefied gas in a pressurized state islocated above an outlet of a return piping of a circulating liquefiedgas at least by a dissolving depth of the pressurizing gas+(plus) aliquid-level movement correction amount.

A second aspect of the cooling system for the superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus characterized in that pressurizing meansfor pressurizing the liquefied gas in the reservoir tank with the sametype of gas as the liquefied gas performs pressurization at apredetermined pressure through a pressure regulating valve from a gascylinder reserving the same type of gas as the liquefied gas at a highpressure.

A third aspect of the cooling system for the superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus characterized in that pressurizing meansfor pressurizing the liquefied gas in the reservoir tank with the sametype of gas as the liquefied gas pressurizes the liquefied gas in thereservoir tank using a piping, which is branched from a piping from anoutlet of the circulating pump to the superconducting power apparatus,and returns to the reservoir tank; and therefore using a dischargepressure of the circulating pump which pumps out the liquefied gas inthe subcooled state from the reservoir tank.

A fourth aspect of the cooling system for the superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus characterized in that pressurizing meansfor pressurizing the liquefied gas in the reservoir tank with the sametype of gas as the liquefied gas comprises an evaporator for evaporatingthe liquefied gas, and a pressure regulating valve for regulating thepressure of the gas, wherein the evaporator and the pressure regulatingvalve are provided at the piping which is branched from a piping from anoutlet of the circulating pump pumping out the liquefied gas in thesubcooled state from the reservoir tank to the superconducting powerapparatus, and returns to the reservoir tank.

A fifth aspect of the cooling system for the superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus characterized in that auxiliary meansfor the pressurizing is further provided so that the same type of gas asthe liquefied gas is supplied from a gas cylinder for thepressurization.

A sixth aspect of the cooling system for the superconducting powerapparatus of the present invention is a cooling system for thesuperconducting power apparatus characterized in that auxiliary meansfor the pressurizing is further provided and the auxiliary means has aheating device arranged at a gas phase portion of the reservoir tank soas to heat and expand a volume of the gas at the phase portion in thereservoir tank.

EFFECT OF THE INVENTION

According to the present invention, a reservoir tank is pressurized bythe same type of a gas as a liquefied gas, air bubbles are not mixed ina liquid nitrogen.

And a cooling system for a superconducting power apparatus in which theliquid nitrogen is smoothly circulated and is excellent in an insulationcharacteristic, can be provided.

Moreover, according to the present invention, since a liquid level of areservoir tank reserving a liquefied gas in a pressurized state islocated above an outlet of a return piping of a circulating liquefiedgas at least by a dissolving depth of pressurizing gas+(plus) aliquid-level movement correction amount, the gas used for thepressurizing the reservoir tank is not liquefied.

And the cooling system for the superconducting power apparatus withoutdropping a pressurized pressure can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph explaining a method for pressurizing a reservoir tankby an outlet pressure of a circulating pump of the present invention.

FIG. 2 is a block diagram of a cooling system explaining an embodiment 1of the present invention.

FIG. 3 is a block diagram in the vicinity of the reservoir tankexplaining an embodiment 2 of the present invention.

FIG. 4 is a block diagram in the vicinity of the reservoir tankexplaining an embodiment 3 of the present invention.

FIG. 5 is a graph showing a relation between a dissolving depth [m] ofpressurizing gas and a pressure decrease rate [%].

FIG. 6 is a diagram explaining a cooling system for a conventionalsuperconducting cable.

DESCRIPTION OF REFERENCE NUMERALS

1 Reservoir tank

1 b Inner container of a reservoir tank

2 Liquid level of liquid nitrogen in a reservoir tank

3 Outlet of liquid nitrogen from a reservoir tank

4, 6, 9 Piping on a feeding side of a liquid nitrogen circulation

5 Circulating pump

5 a Motor of a circulating pump

5 b Extended shaft of a circulating pump

5 c Fin

5 e Vacuum container

7 Heat exchanger of a refrigerating machine

8 Refrigerating machine

10 Entrance of a superconducting power apparatus

11 Superconducting cable

12 Exit of a superconducting cable

13 Piping on a return side of a liquid nitrogen circulation

14 Piping for nitrogen return in a reservoir tank

15 Outlet of a return piping of a liquefied nitrogen gas

16, 18, 20 Branch piping for a pressurization

17 Evaporator

19 Valve

21 External piping for pressurization

22 High-pressure nitrogen cylinder

23 Heater inside a reservoir tank

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A cooling system for a superconducting power apparatus of the presentinvention will be described below in detail referring to attacheddrawings.

The cooling system for the superconducting power apparatus of thepresent invention comprises a reservoir tank for reserving a liquid gas,a circulating pump, a heat exchanger for cooling the liquid gas, and acirculating loop through which the liquefied gas circulates, for coolingthe superconducting power apparatus by circulating the liquefied gas ina subcooled state using the circulating pump, further comprising apressurizing means for pressurizing the reservoir tank with the sametype of gas as the liquid gas, characterized in that a liquid level ofthe reservoir tank reserving the liquefied gas in a pressurized state islocated above an outlet of a return piping of a liquefied nitrogen gasat least by a dissolving depth of pressurizing gas+(plus) a liquid-levelmovement correction amount.

A reason why it is necessary that the liquid level of the reservoir tankreserving the liquefied gas in the pressurized state is located abovethe outlet of a return piping of a liquefied nitrogen gas at least by adissolving depth of the pressurizing gas+(plus) a liquid-level movementcorrection amount is described below.

A relation between a dissolving depth of a pressurizing gas and apressure decrease rate was examined by experiments. FIG. 5 shows therelation between the pressurizing gas dissolving depth [m] and thepressure decrease rate [%].

In FIG. 5, the dissolving depth of the pressurizing gas from a liquidlevel of a reservoir tank (that is, a pressurizing gas dissolving depth)is shown on a lateral axis and a decrease rate per hour of the pressurein the reservoir tank by liquefaction on a longitudinal axis,respectively.

As an experiment condition, an inner volume of the reservoir tank has apressure of 0.3 MPa using a container with a diameter of 1 m and aheight of 1 m.

As a result, as is clear from FIG. 5, a pressure decrease rate isremarkably large up to a dissolving depth of 10 cm, and the decrease ofthe pressurized pressure is still fast up to a dissolving depth ofapproximately 20 cm since a nitrogen gas in a gas phase used forpressurization is condensed into a liquid.

On the other hand, it is found out that if the dissolving depth is keptat 20 cm or more, a pressure decrease amount can be maintained at asmall value of 1% or less.

Actually, since the liquid level is changed by influences oftemperature, pressure and the like of liquid nitrogen besides thedissolving depth of the pressurizing gas, a liquid-level movementcorrection amount should be considered.

Therefore, it is necessary that the liquid level of the reservoir tankreserving the liquefied gas in the pressurized state is located abovethe outlet of a return piping of the liquefied nitrogen gas at least bythe dissolving depth of the pressurizing gas +(plus) the liquid-levelmovement correction amount.

Specifically, an amount of 50 cm or more as the dissolving depth of thepressurizing gas (20 cm)+(plus) a liquid-level movement correctionamount (30 cm) is preferable. The above hardly depends on a containershape of a reservoir tank, and a required depth is substantially thisvalue even if a size of the reservoir tank is different.

Therefore, in a system of the present application, a height which canensure the above-mentioned required depth (50 cm or more is preferable)is required as a container height of the reservoir tank.

As mentioned above, the present invention is to provide a system forcooling a superconducting power apparatus by a liquefied gas which cancirculate liquefied gas in a subcooled state for a long time without aproblem that a gas with a boiling point lower than the liquefied gasused for a pressurization is dissolved in the liquefied gas to cause anunstable factor for a circulation of the liquefied gas or troublerelating to an insulation of the power apparatus.

The pressurizing means in the above-mentioned state comprises apressurization of the liquefied gas in the reservoir tank to apredetermined pressure with the same type of gas as the liquefied gasreserved in the reservoir tank.

In order to prevent a liquefaction of a pressurizing gas from beingcooled by a liquefied gas, a liquid level of a reservoir tank is locatedat a position higher than an outlet of a return piping to a circulatingpump in the reservoir tank, at least by 20 cm or more, preferably by 50cm or more.

Moreover, as pressurizing means in addition to the means forpressurizing with a high-pressure gas cylinder, there is another meansfor the pressurization by returning an output pressure of acirculating-pump, which is higher than the pressure in the reservoirtank.

As specific means using the pressure of the circulating pump outlet,there is means for branching an outlet piping of the circulating pumpfor pumping out a liquid from a reservoir tank and pressurizing andfeeding it to a superconducting power apparatus.

By the branching piping, a part of the liquefied gas is taken out froman outlet of the circulating pump and is gasified by an evaporator andthe gasified gas is returned to the reservoir tank through a pressureregulating valve which is opened or closed according to a pressure tomaintain a pressure in the reservoir tank at a predetermined value.

In order to explain function of the present invention, a case whereliquid nitrogen is used as a liquefied gas is described.

A boiling point of liquid nitrogen at an atmospheric pressure (1.013MPa) is 77K. If this liquid nitrogen is pressurized to 0.3 MPa, theboiling point of the liquid nitrogen becomes 90K or above. Therefore, ifthe liquid nitrogen at 77K is pressurized to 0.3 MPa, the liquidnitrogen is brought into a subcooled state without generating any airbubbles.

The liquid outlet portion of a circulating pump is located at a bottomof a reservoir tank and connected to the circulating pump by a piping.

On the other hand, a return piping of a circulation is connected to thereservoir tank, and an outlet of the return piping of the circulation islocated lower than a liquid level.

The liquefied gas sent out of the circulating pump cools asuperconducting power apparatus and returns to the reservoir tank. Atthat time, since the outlet of the return piping is located at aposition lower than a liquid level, the returning liquefied gas does notcontact a pressurizing gas phase of the reservoir tank and moves to anoutlet of the reservoir tank and further moves to the circulating pumpand recirculates.

In the present invention, the liquid level is set higher than apredetermined height (20 cm) or more, from an outlet of a return pipingof a liquefied nitrogen gas and an outlet of liquid nitrogen to acirculating pump (that is, a predetermined liquefied gas layer isprovided).

And a temperature of the liquid nitrogen, located above the cool liquidnitrogen in a subcooled state at the respective outlets, gradually risestoward a surface of the liquid nitrogen in a reservoir tank and thetemperature of the liquid nitrogen at the surface is substantially thesame as a boiling temperature of a liquefied gas of 0.3 MPa.

In a prior art, there was a problem that if inside of a reservoir tankis pressurized by the same type of gas, the gas is liquefied andconsumed and if the gas can't be supplied in time, then it may cause adrop of a pressure in the reservoir tank.

But in the present invention, by providing the liquefied gas layer inthe reservoir tank, it is found out that the gas is hardly liquefied.

In the present invention, a pressurizing method other than apressurizing by the high-pressure gas cylinder was examined. The methodfor pressurizing by its own piping pressure in the present inventionwill be described referring to FIG. 1.

Firstly, liquid nitrogen is pumped out from an inside of a reservoirtank in an atmospheric pressure state (“a” point in FIG. 1) and is fedby a circulating pump. At an outlet of the circulating pump, the liquidnitrogen flows at a rate of 50 L/min and is pressurized by 0.2 MPa (“b”point) with respect to the pressure at an inlet of the circulating pump.

In using a pressure of the outlet portion of the circulating pump, theliquid nitrogen branched from the outlet piping of the circulating pumpis gasified by an evaporator at the middle point of the piping andreturned to the reservoir tank so as to raise the pressure of thereservoir tank (arrow “c” in FIG. 1).

In response to that, an outlet pressure of the circulating pump israised (arrow “d” in FIG. 1) and is capable of pressurizing a liquid gasin the reservoir tank all the time.

When a pressure in the reservoir tank exceeds (“e” point) an upper-limitset pressure (P2), a valve provided at the piping is closed, and a gassupply to the reservoir tank is stopped.

After that, in the reservoir tank, the nitrogen gas in a gas phase iscooled by the liquid nitrogen which temperature is below a triple pointof the nitrogen gas, and the nitrogen gas in the gas phase is liquefiedinto a liquid nitrogen. And therefore, a pressure in the reservoir tankis decreased (arrow “f”) because a gas volume is decreased by theliquefaction.

When the pressure reaches a lower-limit set pressure (P1) (“g” point),then the valve is opened, and a nitrogen gas is supplied into thereservoir tank by a pressure at a circulating pump outlet again, and apressure in the reservoir tank is pressurized.

Flow of a low-temperature nitrogen gas through a piping might freeze thepiping or valves. Therefore, an evaporator works to prevent it bygasifying the liquid nitrogen and raising the temperature to a roomtemperature.

As the evaporator, there are methods as winding a heater around thepiping, passing the piping through water or the like, mounting a fin tothe piping so as to raise a temperature by heat exchange with an outsideair and the like.

As for the role of a valve, if the gas is continuously supplied merelyby a branched piping from the outlet of the circulating pump, a pressurein a reservoir tank keeps on rising and there is a possibility to exceeda designed pressure of the reservoir tank.

Thus, the valve is closed to stop the pressurization by the gas when thepressure in the reservoir tank exceeds a predetermined pressure, whileit is opened when the pressure falls under the predetermined pressureand resumes the pressurization so as to maintain a constant pressure inthe reservoir tank automatically.

When a capacity of a reservoir tank is large, a large quantity ofnitrogen gas is required for pressurization to a predetermined pressure.Thus, another nitrogen cylinder may be prepared to pressurize thepressure in the reservoir tank to a predetermined pressure.

Also, a method may be used that a heating device such as a heater isarranged at a gas phase portion inside the reservoir tank so as topressurize and expand a gas in the reservoir tank.

The present invention will be described below in detail by means ofembodiments.

Embodiments Embodiment 1

FIG. 2 is a view showing an embodiment of a cooling system for asuperconducting power apparatus of the present invention.

Liquid nitrogen is used as a liquefied gas. The liquid nitrogen isreserved in a reservoir tank 1.

The reservoir tank 1 is in a double-container structure, in which aninsulating material is constructed between a double-containersurrounding an inner container 1 b and maintained in a vacuum state soas to reduce a heat intrusion. Moreover, the reservoir tank is a sealedcontainer so that an inside of the reservoir tank can be pressurized.

At a bottom of the reservoir tank, an outlet 3 of liquid from thereservoir tank connected to a circulating pump is provided, leading toan inlet of the circulating pump 5 by a piping 4 with a diameter of 3cm.

The circulating pump 5 is a swirl rotary pump. A motor 5 a for rotatinga fin 5 c and the fin are connected to each other by an extended shaft 5b of about 50 cm so as to restrict an inflow of heat by a transmission.

Moreover, the fin itself is arranged inside a vacuum container 5 e so asto restrict a heat intrusion from an outside.

The rotary pump in the embodiment of the present invention has arotation frequency of 50 Hz and is capable of a flow rate of 30 L/min asa liquid nitrogen flow rate and discharge a pressure of 0.2 MPa as apressure difference between an inlet and an outlet of the circulatingpump. The pump outlet is connected to a heat exchanger 7 of arefrigerating machine ahead by a piping 6 with a diameter of 3 cm.

The refrigerating machine 8 comprises a GM refrigerating machine or aSterling refrigerating machine and has a heat exchanger connected to alow-temperature head creating frigidness for cooling a circulatingliquid nitrogen to a low temperature.

In the present invention, the Sterling refrigerating machine having arefrigerating capacity of 1 kW is used, and when a liquid nitrogen of 30L/min passes through the heat exchanger cooled by the refrigeratingmachine, 77K at an inlet of the refrigerating machine can be cooled to74K.

The liquid nitrogen cooled by a refrigerating machine is connected to aninlet 10 of a superconducting power apparatus by a piping 9 with adiameter of 3 cm in a water-tight manner.

In the cooling system for cooling the superconducting cable 11 of thisembodiment, the superconducting cable is cooled by passage of liquidnitrogen, which is cooled by the refrigerating machine, in thesuperconducting cable.

The temperature of the liquid nitrogen having cooled the superconductingcable is raised but since the raised temperature is below a boilingpoint, a subcooled state without generation of air bubbles in the liquidnitrogen is maintained.

Therefore, a pressure loss is 0.1 MPa or less, which is sufficientlysmall even in a superconducting cable of 500 m, and the liquid nitrogencan be made to flow stably.

Also, since the liquid nitrogen without generating air bubbles soaksinto an electric insulating layer of a superconducting cable, favorableelectric insulation can be maintained.

The liquid nitrogen going out of an outlet 12 of the superconductingcable returns to a reservoir tank 1 by a piping 13 so as to form acirculation loop.

The reservoir tank 1, the circulating pump 2, the heat exchanger 3 ofthe refrigerating machine, the superconducting cable 4 and the nitrogenpiping connecting these devices to each other are all in adouble-container structure using vacuum heat insulation so as to reduceintruding heat from an outside.

The piping 13 returning to a reservoir tank is a piping 14 from an upperpart of the reservoir tank to a bottom thereof for returning the liquidto the reservoir tank from an outlet 15 of a return piping at a bottomportion of the tank. An outlet 3 of liquid nitrogen connected to thecirculating pump is also located at a bottom of the reservoir tank.

During circulation, liquid nitrogen of the reservoir tank is reserved sothat a liquid level 2 is located at a position higher than a position ofthe outlet 15 at least by 20 cm.

By a method for pressurizing a reservoir tank by an outlet pressure of acirculating pump of the present invention, a stainless piping 16 with adiameter of 6 mm is branched from a piping 6 of the pump outlet and thepressure of the liquid nitrogen there is taken out.

The liquid nitrogen flowing inside the piping 16 passes through anevaporator 17 after going out of a vacuum container of a circulatingpump and all the liquid nitrogen is changed to a room temperaturenitrogen gas.

As an evaporator, a 6 mm piping made of copper is wound in a 6 m coilconfiguration inside of a hot water container, is used in thisembodiment. And the evaporator is soaked in hot water to raise atemperature of liquid nitrogen inside.

Any evaporators may be used only if liquid nitrogen inside can be madeinto a room-temperature gas, including a method in which a heater iswound around an outside of a coil, for example, to raise a temperatureof the liquid nitrogen by heat generated by a heater or a method inwhich a fin is mounted on a piping for warming the liquid nitrogen by aheat exchange with an outside air.

A piping 18 coming out of the evaporator 17 is provided with a valve 19having a pressure control function to flow a gas when an outlet pressurefalls below a predetermined pressure and to stop the gas at apredetermined pressure or above. A piping 20 coming out of the valve 19is mounted at an upper part of a reservoir tank so as to pressurize thereservoir tank.

Since the pipings 18, 20 after passage through the evaporator 17 are ata room temperature, it is not necessary to make them into an insulatingstructure, but it is preferable on appearance that the piping 16 fromthe circulating pump outlet to the evaporator is surrounded by aninsulating material such as a foaming urethane or the like to prevent afrost on the piping 16.

If a valve operating at a low temperature is used for the valve 19, apositions of the valve 19 and the evaporator 17 can be switched, but avalve which is used in a low temperature is more expensive than that fora normal temperature and that is not an economical arrangement.

In this embodiment, the piping 16 for taking out pressure of the outletof the circulating pump, branches from the piping 6 at the pump outlet,but branching from a piping anywhere higher in the pressure than in thereservoir tank can achieve an object of the present invention, whetherit branches from the piping 9 at an outlet of a heat exchanger of arefrigerating machine or from an inlet portion 10 of a superconductingapparatus.

Thus, the pump outlet collectively refers to all portions of the pipingwhich is downstream of the circulating pump outlet, not only to animmediate vicinity of the pump outlet.

Embodiment 2

In the embodiment 1, a case where a circulating pump is located outsidea reservoir tank is described, but the present invention can be alsoapplied to a case where the circulating pump is located inside thereservoir tank.

FIG. 3 shows a view of a part of another mode of a cooling system for asuperconducting power apparatus of the present invention. That is, FIG.3 shows an extraction view of a reservoir tank portion of the coolingsystem to explain this embodiment.

In a circulating pump 5, a fin portion 5 c for feeding liquid nitrogenis provided in a liquid of a reservoir tank, and a rotation of a motor 5a is transmitted by an extended shaft 5 b.

The liquid nitrogen is pumped out from the reservoir tank, passesthrough a piping 6, goes out of the reservoir tank and is connected to arefrigerating machine for cooling the liquid nitrogen.

In this case, a piping for pressurization is mounted at a portion of thepiping 6, coming out of the reservoir tank, and returns to the reservoirtank through an evaporator 17 and a valve 19 as in the embodiment 1.

Embodiment 3

In the embodiment 1, a pressurizing means for the reservoir tank is onlyby a gas from a circulating pump outlet. In this case, since the pipingis as thin as 6 mm and has only a discharge pressure of the circulatingpump, the gas supply is small and it takes an extremely long time toreach a predetermined pressure in the reservoir tank.

Particularly, if the reservoir tank is large, it takes several tens ofhours. Then, as shown in FIG. 4, an external piping 21 is mounted to thereservoir tank as auxiliary means for supplying gas from a high-pressurenitrogen cylinder 22 or nitrogen curdle.

Moreover, since liquefaction is promoted when a gas phase portion insidethe reservoir tank is cooled to a low temperature, a heater 23 may bearranged in a gas phase portion to restrain from the liquefaction.

INDUSTRIAL APPLICABILITY

According to the present invention, a cooling system for asuperconducting power apparatus can be provided.

In the cooling system, a liquefied gas in a subcooled state iscirculated for a long time, without an unstable factor of a circulationof the liquefied gas by dissolution of a gas with a boiling point lowerthan the liquefied gas used for pressurization, and without a troublerelating to an insulation of an electric apparatus.

1. A cooling system for a superconducting power apparatus comprising: areservoir tank for reserving a liquefied gas; a circulating pump; a heatexchanger for cooling the liquefied gas; a circulation loop throughwhich the liquefied gas is circulated, in which the superconductingpower apparatus is cooled by circulating the liquefied gas in asubcooled state using the circulating pump; and further comprisingpressurizing means for pressurizing the liquefied gas in the reservoirtank with the same type of gas as the liquefied gas, wherein a liquidlevel in the reservoir tank for reserving the liquefied gas in apressurized state is located above an outlet of a return piping of acirculating liquefied nitrogen gas at least by a dissolving depth of thepressurizing gas+(plus) a liquid level movement correction amount. 2.The cooling system for the superconducting power apparatus according toclaim 1, wherein the pressurizing means for pressurizing the liquefiedgas in the reservoir tank with the same type of gas as the liquefied gasperforms pressurization at a predetermined pressure through a pressureregulating valve from a gas cylinder reserving the same type of gas asthe liquefied gas at a high pressure.
 3. The cooling system for thesuperconducting power apparatus according to claim 1, wherein thepressurizing means for pressurizing the liquefied gas in the reservoirtank with the same type of gas as the liquefied gas comprising: apiping, which is branched from a piping from an outlet of thecirculating pump to the superconducting power apparatus, and returns tothe reservoir tank; and therefore using a discharge pressure of thecirculating pump which pumps out the liquefied gas in the subcooledstate from the reservoir tank.
 4. The cooling system for thesuperconducting power apparatus according to claim 3, wherein thepressurizing means for pressurizing the liquefied gas in the reservoirtank with the same type of gas as the liquefied gas comprising: anevaporator for evaporating the liquefied gas; and a pressure regulatingvalve for regulating the pressure of the gas, wherein the evaporator andthe pressure regulating valve are provided at the piping which isbranched from a piping from an outlet of the circulating pump pumpingout the liquefied gas in the subcooled state from the reservoir tank tothe superconducting power apparatus, and returns to the reservoir tank.5. The cooling system for the superconducting power apparatus accordingto claim 3, wherein auxiliary means for the pressurizing means isfurther provided so that the same type of gas as the liquefied gas issupplied from a gas cylinder for the pressurization.
 6. The coolingsystem for the superconducting power apparatus according to claim 1,wherein the auxiliary means for the pressurizing is further provided andthe auxiliary means has a heating device arranged at a gas phase portionof the reservoir tank so as to heat and expand a volume of the gas atthe phase portion in the reservoir tank.
 7. The cooling system for thesuperconducting power apparatus according to claim 4, wherein auxiliarymeans for the pressurizing means is further provided so that the sametype of gas as the liquefied gas is supplied from a gas cylinder for thepressurization.
 8. The cooling system for the superconducting powerapparatus according to claim 2, wherein the auxiliary means for thepressurizing is further provided and the auxiliary means has a heatingdevice arranged at a gas phase portion of the reservoir tank so as toheat and expand a volume of the gas at the phase portion in thereservoir tank.
 9. The cooling system for the superconducting powerapparatus according to claim 3, wherein the auxiliary means for thepressurizing is further provided and the auxiliary means has a heatingdevice arranged at a gas phase portion of the reservoir tank so as toheat and expand a volume of the gas at the phase portion in thereservoir tank.
 10. The cooling system for the superconducting powerapparatus according to claim 4, wherein the auxiliary means for thepressurizing is further provided and the auxiliary means has a heatingdevice arranged at a gas phase portion of the reservoir tank so as toheat and expand a volume of the gas at the phase portion in thereservoir tank.
 11. The cooling system for the superconducting powerapparatus according to claim 5, wherein the auxiliary means for thepressurizing is further provided and the auxiliary means has a heatingdevice arranged at a gas phase portion of the reservoir tank so as toheat and expand a volume of the gas at the phase portion in thereservoir tank.
 12. The cooling system for the superconducting powerapparatus according to claim 7, wherein the auxiliary means for thepressurizing is further provided and the auxiliary means has a heatingdevice arranged at a gas phase portion of the reservoir tank so as toheat and expand a volume of the gas at the phase portion in thereservoir tank.